Hydrofluoric acid is useful in such diverse fields as isoparaffin-olefin alkylation, fluorination, semiconductor manufacture, steroid synthesis, tantalum recovery, and xylene separation.
Industrial isoparaffin-olefin alkylation processes have historically used concentrated hydrofluoric acid catalysts under relatively low temperature conditions. The acid strength is preferably maintained at 88 to 94 weight percent by the continuous addition of fresh acid and the continuous withdrawal of spent acid. As used herein, the term "concentrated hydrofluoric acid" refers to an essentially anhydrous liquid containing at least about 85 weight percent HF.
Alkylation is a reaction in which an alkyl group is added to an organic molecule. Thus an isoparaffin can be reacted with an olefin to provide an isoparaffin of higher molecular weight. Industrially, the concept depends on the reaction of a C.sub.2 to C.sub.5 olefin with isobutane in the presence of an acidic catalyst producing a so-called alkylate. This alkylate is a valuable blending component in the manufacture of gasolines due not only to its high octane rating but also to its sensitivity to octane-enhancing additives. For a survey of hydrofluoric acid catalyzed alkylation, see 1 Handbook of Petroleum Refining Processes 23-28 (R. A. Meyers, ed., 1986).
Recently, more stringent environmental regulations have prompted a new look at methods of storing and processing hydrofluoric acid. Specifically, researchers have investigated possible solvents which could be used to dilute the hydrofluoric acid (thus rendering it safer) while preserving its commercial useful characteristics. Tetrahydrothiophene-1,1,-dioxide (also referred to herein as sulfolane) has been found to be a useful additive for hydrofluoric acid in isoparaffin-olefin alkylation.
Dilute solutions of water and hydrofluoric acid are highly corrosive toward carbon steel. Neat hydrofluoric acid is essentially noncorrosive toward carbon steel, and it is industry practice to handle and store neat hydrofluoric acid using carbon steel equipment. Neat tetrahydrothiophene-1,1-dioxide (sulfolane) is similarly relatively noncorrosive toward carbon steel. Surprisingly, mixtures of hydrofluoric acid and tetrahydrothiophene-1,1-dioxide are highly corrosive. Carbon steel process equipment would have a projected useful life of no more than a few months in the presence of mixtures of hydrofluoric acid and tetrahydrothiophene-1,1-dioxide.
Carbon steel typically loses mass (corrodes) when it is the anode in a galvanic cell. But if the carbon steel is connected to an appropriate anode, the carbon steel becomes the cathode, and the anode corrodes (is sacrificed) to preserve the carbon steel. For example, residential water heaters often contain sacrificial zinc anodes which are electrically coupled to the vessel shell. The zinc is referred to as the sacrificial anode because, in its role as the anode of a galvanic cell, the zinc loses mass in the corrosion reaction and is sacrificed to save the carbon steel.
Carbon steel above-ground storage tanks can contain sacrificial anodes which are buried in the ground just below the tank floor. The sacrificial anode, which is typically zinc or magnesium, is electrically connected to the tank floor by a cable to prevent corrosion damage to the tank.
Other materials of construction, in contrast, become more resistant to corrosion when they become the anode of a galvanic cell. Impressed current, or anodic protection, prevents long-term damage to certain austenitic stainless steels in contact with sulfuric acid. These austenitic stainless alloys corrode to a point, and then appear to form a tenacious protective film which prevents further attack.
It is generally accepted in the industry that raising the potential of a carbon steel structure (making it more anodic) with respect to a corrosive solution accelerates corrosive attack.